The Swagger Inflector library has the ExampleBuilder class exactly for this purpose. It lets you generate JSON, XML and YAML examples from models in an OpenAPI (Swagger) definition.

OpenAPI 2.0 example

To work with OpenAPI 2.0 (swagger: '2.0') definitions, use Swagger Java libraries 1.x.

import io.swagger.parser.SwaggerParser;
import io.swagger.models.*;
import io.swagger.inflector.examples.*;
import io.swagger.inflector.examples.models.Example;
import io.swagger.inflector.processors.JsonNodeExampleSerializer;
import io.swagger.util.Json;
import io.swagger.util.Yaml;
import java.util.Map;
import com.fasterxml.jackson.databind.module.SimpleModule;

...

// Load your OpenAPI/Swagger definition
Swagger swagger = new SwaggerParser().read("http://petstore.swagger.io/v2/swagger.json");

// Create an Example object for the Pet model
Map<String, Model> definitions = swagger.getDefinitions();
Model pet = definitions.get("Pet");
Example example = ExampleBuilder.fromModel("Pet", pet, definitions, new HashSet<String>());
// Another way:
// Example example = ExampleBuilder.fromProperty(new RefProperty("Pet"), swagger.getDefinitions());

// Configure example serializers
SimpleModule simpleModule = new SimpleModule().addSerializer(new JsonNodeExampleSerializer());
Json.mapper().registerModule(simpleModule);
Yaml.mapper().registerModule(simpleModule);

// Convert the Example object to string

// JSON example
String jsonExample = Json.pretty(example);
System.out.println(jsonExample);

// YAML example
String yamlExample = Yaml.pretty().writeValueAsString(example);
System.out.println(yamlExample);

// XML example (TODO: pretty-print it)
String xmlExample = new XmlExampleSerializer().serialize(example);
System.out.println(xmlExample);

OpenAPI 3.0 example

For an OpenAPI 3.0 example, see this answer. You need version 2.x of Swagger Java libraries, and update the imports and class names appropriately, e.g. change io.swagger.parser.SwaggerParser to io.swagger.v3.parser.OpenAPIV3Parser and so on.

You can use this JSON-to-OpenAPI schema converter:
https://roger13.github.io/SwagDefGen/

(GitHub project)

I haven't used it personally though, so I'm not sure how good it is.

Since OpenAPI uses a subset of JSON Schema, you could also use one of the JSON Schema generators, however you may need to manually tweak the generated definition to make it OpenAPI-compatible.